Optical communications systems, as well as other optical technologies, require apparatus that manipulates optical signals that are in the form of light rays. One of the best known apparatus for such manipulation is the silica based optical fiber which is widely used for both short and long distance optical transmission. Additionally, other well known apparatus include couplers and multiplexers that have been developed to couple one or more optical signals into one or more optical fibers or waveguides. Some applications for transmission of optical signals also desirably use non-reciprocal apparatus or devices, where the propagation characteristics of light within the apparatus depends upon the direction of light propagation within the apparatus.
One type of such non-reciprocal apparatus, commonly referred to as an optical isolator, permits light to pass through the apparatus in one direction but not in the reverse direction. Another type of such non-reciprocal apparatus is termed an optical circulator. An optical circulator has 3 or more ports which permit light to pass from a first to a second port, for example, but not from the second port to the first port. Instead light entering the second port passes to a third port. If there is a fourth port, then light entering the third port exits at this fourth port. Generalizing, port n .fwdarw. port (n+1) if ports n and (n+1) exist. A partial circulator is a circulator as described above but where port n.sub.max does not lead to port 1. A complete or full circulator couples light entering port n.sub.max to port 1. An isolator may be thought of as a two port partial circulator. However, the term isolator is the preferred term.
Many types of optical devices, including circulators and isolators, have been developed for commercial applications. For many applications, such a device should be polarization independent to the external world; that is, device operation should not depend upon the polarization of the incoming light. For example, Fujii in Journal of Lightwave Technology, Vol. 10, pp. 1226-1229, September 1992, describes a polarization independent apparatus that as stated can be used as an optical circulator. Although stated to be useful over a wide range of wavelengths, preferred operation of the apparatus depends upon precise orientation of the components with respect to each other. Another optical circulator is described in U.S. Pat. No. 5,204,771 issued to Koga on Apr. 20, 1993. The essence of the Koga circulator appears to be the use of a birefringent plate followed by non-reciprocal optical rotators. The birefringent plate splits the incoming beam into two parallel beams, and the optical rotators change the polarization of each of the two beams by 45 degrees so that there are two parallel beams with orthogonal polarization.
Consideration of the devices described in the previous paragraph reveals aspects that make their use disadvantageous in some situations. As mentioned, the Fujii circulator depends upon precise relative orientation of the components. This orientation may be difficult to achieve in practice and still more difficult to maintain for extended periods of time. The Koga device is extremely complicated. The complexity arises not only because there are many components, where the individual components are made up of multiple connected plates, but also because the large number of components necessarily has a large number of surfaces. Any one of these surfaces can produce undesired reflections. The devices must be designed to either eliminate the reflections or to compensate for them. Additionally, the optical rotators must be precisely aligned so that one beam passes through the top half of the rotator and the other beam passes through the bottom half of the rotator. The small beam sizes and small separation of the beams will likely make this difficult. Accordingly, there is a need for an optical circulator device which is simpler to manufacture and maintain than those devices found in the prior art.